1. Field of the Invention
The present invention relates to new coating compositions based on amines, aldimines, and isocyanates. This coating composition can be cured at ambient conditions or can be cured by baking. The coating composition optionally comprises of pigments and well known paint additives such as fillers, rheology control agents, dispersing agents, solvents, etc. The present invention doesn't consist of any hydroxyl functional compounds. The coating is especially useful for automotive refinish market.
2. Description of Prior Art
Common coating systems which have been used for many years in the coatings industry are polyurethane systems. These systems contain hydroxyl-functional polyols which react with isocyanates giving excellent film properties including durability, toughness, and solvent resistent films. These systems have been popular for automotive coatings, especially automotive refinish. The reason for the popularity in automotive refinish coatings is due to the rate of the reaction between the polyol and isocyanate. This reaction takes place very slow and allows for a long pot life. Pot life being the time after a catalyzed paint sample is mixed when it is still low enough in viscosity that it may still be sprayed. Unfortunately polyurethane coatings take a long time to dry. The reaction can be increased by the addition of an organotin catalyst, but when the catalyst is added, the pot life and dry time both decrease. The desired application would be a combination between long pot life and short dry time.
With increasing Volatile Organic Content (VOC) restrictions, it is difficult to get a balance of pot life and dry time with polyurethane chemistry. The main problem is the hydroxyl functional polyols are high molecular weight resins, which are viscous materials and require a lot of organic solvents in order to reduce to sprayable viscosities. It is also difficult to get the VOC low enough to be compliant with environmental restrictions. By using reactive diluents in combination with the polyol and isocyanates, lower VOC can be achieved. An excellent example of this is U.S. Pat. No. 5,214,086 Mormile et al. (assigned to BASF Corp.). In this patent, the use of diketimines, dialdimines, and hindered diamines as reactive diluents, helps to lower the viscosity and increase the solids, thus enabling coating compositions to reach VOC limits. However, more stringent VOC limits are being set, so it is necessary to try and achieve even lower VOCs. The low molecular weight hydroxyl functional polyols are difficult to use in order to reach these VOC limits. In addition, these polyols react too fast and a sufficient pot life is hard to achieve.
The following patents demonstrate the removal of all or most of the hydroxyl functional polyols and replacing these polyols with other active hydrogen compounds:
U.S. Pat. No. 5,126,170 describes a coating composition based on a two component system, of a polyisocyanate component and an isocyanate-reactive component containing partly or entirely certain secondary polyamines. The use of this coating exhibits fast dry times, but inadequate pot life, i.e., the viscosity of the system increases too rapidly prior to application of the coating compositions to a substrate.
U.S. Pat. No. 5,243,012 uses the same chemistry as the above mentioned patent, but provides longer pot life without significantly increasing the dry time of the coating composition by the use of tin catalyst. The present invention doesn't require the use of tin catalyst. Instead, the addition of aldimine helps to achieve an adequate pot life and dry time. Contrary to the previous patent, the addition of tin to the present invention catalyzes the reaction, making the pot life shorter.
U.S. Pat. Nos. 4,469,831; 4,720,535; 4,504,647; 5,010,161; 5,087,661; all deal with the reaction of active hydrogens from ketimines, aldimines, or oxazolidines with isocyanate. Most of these deal with aldimine or polyaldimine reacted with isocyanates in the presence of moisture in order for the reaction to proceed quickly. The present invention provides adequate dry time without the presence of moisture and uses secondary amines, aldimines, and isocyanates.
U.S. Pat. No. 4,469,831 Bueltjer et al., 4,720,535 Schleier et al., and 4,853,454 Merger et al. (all assigned to BASF AG) all describe compositions of active hydrogen, aldimine functional materials reacted with isocyanate functional prepolymers. The rate of the reaction of this chemistry can be accelerated by the use of acid catalysts. When the aldimine in combination with the acid, is exposed to moisture, the acid helps to hydrolyze the dialdimine. In the present invention, the acid helps to catalyze the reaction of the secondary amine and aldimine with the isocyanate. The presence of moisture is not necessary for this reaction to proceed.
U.S. Pat. No. 3,682,867 describes the use of secondary amines reacted with isocyanates to form polyurea polymers. These new polymers can be mixed with isocyanate and have good stability in the absence of moisture. This system can then be mixed with primary amine groups converted to aldimine or ketimine groups. The polyurea polymers are necessary for the elimination of the problem of gel particle formation which often occurs in the mixing of polyisocyanates with ketimine or aldimine. The importance is that the polyurea needs to be formed first. In the present invention, there is no need for a polyurea prepolymer. The aldimine is compatible with the isocyanate and will not gel.
The present invention deals with the use of secondary amines, aldimines, and isocyanates to achieve low VOC coating compositions. VOC of less than 1.5 Lbs/Gallon can be obtained and still balance pot life and dry times with or without the presence of moisture. Acid catalysts are optional in order to speed up the reaction rate.